1. Field of the Invention
The invention relates to refractive-index sensors and, particularly, to a refractive-index sensor based on photonic crystals.
2. Description of Related Art
Recently, there has been interest in the use of photonic crystals to measure refractive-index change for sensing applications, because of the unique light-confinement mechanism provided by the photonic band-gap. Photonic crystals provide the potential for a high quality factor (high-Q) microcavity with small sensing area. For example, the sensing area may be 10□ μm2, which requires only a very small volume (e.g., 10−15 liters) sample. These advantages make the photonic crystal an attractive candidate for use in small sample measurement. Thus, refractive-index sensors based on photonic crystals have been developed extensively.
As shown in FIG. 4, a conventional refractive-index sensor 100 based on a photonic crystal includes a substrate 102, a plurality of air holes 104, a first waveguide 108, and a second waveguide 110. The air holes 104 are etched through the substrate 102. The photonic crystal microcavity is formed by introducing one hole 106 with a diameter different from that of the air holes 104. The first waveguide 108 and the second waveguide 110 are each comprised in a side half of the substrate 102. The waveguides 108, 110 are configured to couple light in and out of the photonic crystal microcavity. The refractive-index sensor 100 is capable of detecting a change in refractive index of 0.002. Additionally, the refractive-index sensor 100 can measure a sample having a refractive index (n) from n=1.0 to n=1.5.
However, there is ongoing demand for a refractive-index sensor to provide high light transmission and improved sensitivity. A new refractive-index sensor that can meet this demand is desired.